With the maturation of the computer and surrounding technologies, vast amounts of complex, mixed traffic types are transmitted through synchronous optical networks (SONETs). The SONET standard is described in the American National Standards Institute (ANSI) standards T1.105 and T1.106 and in the Bellcore Technical Recommendations TR-TSY-000253.
The basic building block of SONET networks is the SONET ring connection. FIG. 1 illustrates a basic SONET ring connection. SONET switch 100 and SONET switch 150 receive optical signals from various devices (not shown in FIG. 1). SONET switch 100 and SONET switch 150 can be coupled to other SONET switches, or other devices that communicate data using optical signals.
SONET switch 100 and SONET switch 150 communicate using two sets of unidirectional signaling pairs. In general, half of the data traffic between switches travels over one of the signaling pairs and the other half of the traffic travels over the other signaling pair. SONET switches communicate according to a predetermined protocol, and at a predetermined bit rate. Telecommunications (Telco) SONET systems have been designed and implemented using digital signaling (DS) technology, which is well known in the art.
For proper transmission of data traffic between the sonet switches, the destination switch receives data at the same rate as the source switch. Therefore, SONET switch 100 and SONET switch 150 typically include system clock units to facilitate the synchronization of traffic. The system clock units provide a reference clock source for transactions received and transmitted from a switch. SONET switch 100 and SONET switch 150 each include backup clock units for redundancy in the event a failure occurs at the primary clock unit.
Often, a problem in synchronization may occur upon switching from a primary clock unit to a backup clock unit. Moreover, it may take several seconds for resynchronization to occur. Such a large delay in synchronization may cause entire data packets received at or transmitted from a switch to be garbled or lost. The loss of data packets forces retransmission, which slows the network. Therefore, what is desired is a method and apparatus for fast resynchronization upon switching from a primary clock unit to a backup clock unit.